The long-term goal of these studies is to add to our basic knowledge of the mechanism of gene regulation. Currently there are two major objectives: 1) We would like to know whether the order of replication of individual genes can be observed on a molecular level and whether different cell types exhibit different patterns in the order of DNA replication. This can be ascertained by measuring the time of replication of a single gene in cells in which it is active and in cells in which it is inactive. We plan to look first at the hemoglobin gene. 2) To continue our studies on the DNA of animal cells to increase our knowledge of the mechanism and significance of different states of condensed chromatin. These include studies on: a) DNA replication early during the DNA synthetic period. b) Chromosome structure. c) Chromosome fractionation. d) Single-stranded and other unusual DNA species observed in isolated mammalian nuclear DNA. BIBLIOGRAPHIC REFERENCES: Balazs, I., and Schildkraut, C. L. DNA Replication in Synchronized Cultured Mammalian Cells. VI. The Temporal Replication of Ribosomal Cistrons in Three Synchronized Cell Lines, Exp. Cell Res., 101, 307 (1976). Maio, J.J., and Schildkraut, C. L., Mammalian Metaphase Chromosomes, in Methods in Chromosomal Protein Research, G. Stein, J. Stein and L. Kleinsmith (Editors). Academic Press (1977) in press.